I found this design on the net while searching for an accurate and reliable yet cheap proximity sensor circuit.
The circuit may be understood with the help of the following description:
Referring to the below shown infrared (IR) motion detector circuit, we see the design consisting of two main stages, one involving the IC LM567 while the other with the IC555.
Basically the IC LM567 becomes the heart of the circuit which solely performs the functions of the generating/transmitting the IR frequency and also detecting the same.
Moreover the IC has an internal phase locked loop circuitry which makes it highly reliable with frequency detecting circuit applications.
It means once it reads and latches to a given frequency, its detection feature gets locked to that frequency and therefore any other stray disturbance no matter how strong it may be doesn't influence or rattle its functioning.
An internal oscillator frequency determined by R3, C2 feeds the IR diode D274 via a current controlled stage consisting T1, R2. This frequency decides the center frequency of the chip.
With the above conditions the IC gets set and centered at the above frequency generating a constant high at its output pin#8.
Input pin#3 of the IC waits to receive a frequency which may be exactly equal to the above "centered" frequency of the IC.
The IR receiver or the sensor connected across pin#3 of the IC is positioned exactly for this purpose.
As soon the IR beam from the LD274 finds an obstacle, its beam gets reflected and falls on the appropriately positioned detector diode BP104.
The IR frequency from the LD274 now passes to the input pin#3 of the IC, since this frequency will be exactly same to the set center frequency of the IC, the IC recognizes this and instantly switches its output from high to LOW.
The above low trigger at pin#2 of the IC 555 which is configured as a monostable in turn switches its output high, causing the connected alarm to blow.
The above condition persists for so long as the interruption from of the IR sensor/ detector stays and allows the beams to get reflected. With the inclusion of R9 and C5, the output of IC555 exhibits a certain delay off condition for the connected buzzer even after the motion or the obstacle moves away.
For adjusting the delay-off effect, R9 and C5 may be tweaked as per preference.
The circuit may also be used as a proximity detector and obstacle detector circuit.